Cybersecurity researchers have disclosed two new assault strategies on licensed PDF paperwork that might probably allow an attacker to change a doc’s seen content material by displaying malicious content material over the certiﬁed content material with out invalidating its signature.
“The assault concept exploits the pliability of PDF certification, which permits signing or including annotations to licensed paperwork beneath completely different permission ranges,” said researchers from Ruhr-College Bochum, who’ve systematically analyzed the safety of the PDF specification through the years.
The findings have been offered on the forty second IEEE Symposium on Safety and Privateness (IEEE S&P 2021) held this week.
The 2 assaults — dubbed Evil Annotation and Sneaky Signature attacks — hinge on manipulating the PDF certification course of by exploiting flaws within the specification that governs the implementation of digital signatures (aka approval signature) and its extra versatile variant known as certification signatures.
Certification signatures additionally enable completely different subsets of modifications on the PDF doc based mostly on the permission stage set by the certifier, together with the power to jot down textual content to particular type fields, present annotations, and even add a number of signatures.
The Evil Annotation Assault (EAA) works by modifying a certiﬁed doc that is provisioned to insert annotations to incorporate an annotation containing malicious code, which is then despatched to the sufferer. Alternatively, the thought behind the Sneaky Signature assault (SSA) is to control the looks by including overlaying signature components to a doc that permits filling out type fields.
“By inserting a signature area, the signer can outline the precise place of the sphere, and moreover its look and content material, the researchers stated. “This flexibility is critical since every new signature might comprise the signer’s data. The data could be a graphic, a textual content, or a mix of each. Nonetheless, the attacker can misuse the pliability to stealthily manipulate the doc and insert new content material.”
In a hypothetical assault state of affairs detailed by the lecturers, a certifier creates an authorized contract with delicate data whereas enabling the choice so as to add additional signatures to the PDF contract. By making the most of these permissions, an attacker can modify the contents of the doc, say, to show an Worldwide Financial institution Account Quantity (IBAN) beneath their management and fraudulently switch funds, because the sufferer, unable to detect the manipulation, accepts the tampered contract.
15 of 26 PDF functions evaluated by the researchers, counting Adobe Acrobat Reader (CVE-2021-28545 and CVE-2021-28546), Foxit Reader (CVE-2020-35931), and Nitro Professional, have been discovered weak to the EAA assault, enabling an attacker to alter the seen content material within the doc. Soda PDF Desktop, PDF Architect, and 6 different functions have been recognized as vulnerable to SSA assaults.
To fend off such assaults, the researchers advocate prohibiting FreeText, Stamp, and Redact annotations in addition to making certain that signature fields are arrange at outlined areas within the PDF doc previous to certification, alongside penalizing any subsequent addition of signature fields with an invalid certification standing. The researchers have additionally created a Python-based utility known as PDF-Detector, which parses licensed paperwork to focus on any suspicious components discovered within the PDF doc.